The invention generally relates to a steam generator with a nitrogen removal device for fuel gas and with a combustion chamber for fossil fuel which is followed on the fuel-gas side, via a horizontal gas flue and a vertical gas flue, by the nitrogen removal device for fuel gas.
In a power plant with a steam generator, the fuel gas generated during the combustion of a fossil fuel is used for the evaporation of a flow medium in the steam generator. For the evaporation of the flow medium, the steam generator has evaporator tubes, of which the heating by fuel gas leads to an evaporation of the flow medium carried in them. The steam provided by the steam generator may, in turn, be provided, for example, for a connected external process, or for driving a steam turbine. When the steam drives a steam turbine, a generator or a working machine is normally operated via the turbine shaft of the steam turbine. Where a generator is concerned, the current generated by the generator may be provided for feeding into an interconnected and/or island network.
The steam generator may in this case be designed as a continuous-flow steam generator. A continuous-flow steam generator is known from the paper xe2x80x9cVerdampferkonzepte fxc3xcr Benson-Dampferzeugerxe2x80x9d [xe2x80x9cEvaporator concepts for Benson Steam Generatorsxe2x80x9d] by J. Franke, W. Kxc3x6hler and E. Wittchow, published in VGB Kraftwerkstechnik 73 (1993), No. 4, p. 352-360. In a continuous-flow steam generator, the heating of steam generator tubes, provided as evaporator tubes, leads to an evaporation of the flow medium in the steam generator tubes in a single pass.
Steam generators are usually designed with a combustion chamber in a vertical form of construction. This means that the combustion chamber is designed for the heating medium or fuel gas to flow through in an approximately vertical direction. In this case, the combustion chamber may be followed, on the fuel-gas side, by a horizontal gas flue, a deflection of the fuel-gas stream into an approximately horizontal flow direction taking place at the transition from the combustion chamber into the horizontal gas flue. In general, however, because of the thermally induced changes in length of the combustion chamber, combustion chambers of this type require a framework on which the combustion chamber is suspended. This necessitates a considerable technical outlay in terms of the production and assembly of the steam generator, this outlay being higher, the greater the overall height of the steam generator is.
A particular problem is the design of the containment wall of the gas flue or combustion chamber of the steam generator with regard to the tube-wall or material temperatures which occur there. In the subcritical pressure range to about 200 bar, the temperature of the containment wall of the combustion chamber may be determined by the height of the saturation temperature of the water. This is achieved, for example, using evaporator tubes which have a surface structure on their inside. Consideration is given, in this respect, to internally ribbed evaporator tubes, of which the use in a continuous-flow steam generator is known, for example, from the abovementioned paper. These ribbed tubes, that is to say tubes with a ribbed inner surface, have particularly good heat transmission from the tube inner wall to the flow medium.
To reduce the nitrogen oxides in the fuel gas of the fossil fuel, the method of selective catalytic reduction, which is known as the SCR method, may be used. In the SCR method, nitrogen oxides (NOx) are reduced to nitrogen (N2) and water (H2O) with the aid of a reducing agent, for example ammonia, and a catalyst.
In a steam generator designed for an SCR method, a nitrogen removal device for fuel gas, with a catalyst, is conventionally arranged downstream of the fuel-gas duct, which is designed as a convection flue and where the fuel gas normally has a temperature of about 320 to 400xc2x0 C. The catalyst of the nitrogen removal device for fuel gas serves to initiate and/or maintain a reaction between the reducing agent introduced in the fuel gas and the nitrogen oxides of the fuel gas. The reducing agent required for the SCR method is in this case usually injected, together with air as a carrier stream, into the fuel gas flowing through the gas flue. However, as a rule, the nitrogen oxide emission of the steam generator depends on the type of fossil fuel burnt. Therefore, in order to adhere to the legally prescribed limit values, the reducing agent quantity to be injected is normally varied as a function of the fossil fuel used.
However, a nitrogen removal device for fuel gas, arranged downstream of the convection flue on the outlet side, requires a considerable outlay in structural and production terms for the respective steam generator. This is because the nitrogen removal device has to be arranged in the steam generator in a place where it can exert a particularly high purifying effect on the fuel gas in all the operating states of the steam generator. This is normally the case where the fuel gas has a temperature in the range of about 320 to 400xc2x0 C. Moreover, the outlay in terms of the production of a steam generator increases when the latter has, as well as conventional components, a nitrogen removal device in addition.
An object on which the invention is based is, therefore, to specify a fossil-fired steam generator of the abovementioned type, which requires a particularly low outlay in structural and production terms and in which a purification of the fuel gas of the fossil fuel is ensured particularly reliably, before these leave the steam generator on the outlet side.
This object is achieved, according to the invention, in that the combustion chamber of the steam generator includes a number of burners arranged level with the horizontal gas flue, the vertical gas flue being designed for an approximately vertical flow of the flue gas from the bottom upward and the nitrogen removal device for fuel gas being designed for an approximately vertical flow of the fuel gas from the top downward.
The invention proceeds from the notion that a steam generator capable of being erected at a particularly low outlay in production and assembly terms should have a suspension structure capable of being produced in a simple manner. A framework, to be erected at a comparatively low technical outlay, for the suspension of the combustion chamber may at the same time be accompanied by a particularly low overall height of the steam generator. A particularly low overall height of the steam generator can be achieved by the combustion chamber being designed in a horizontal form of construction. For this purpose, the burners are arranged, level with the horizontal gas flue, in the combustion chamber wall. Thus, when the steam generator is in operation, the flue gas flows through the combustion chamber in an approximately horizontal direction.
For a particularly reliable purification of the fuel gas of the fossil fuel, the nitrogen removal device for fuel gas may be arranged downstream of the vertical gas flue on the outlet side. To be precise, downstream of the vertical gas flue on the outlet side, the fuel gas has temperatures at which a purification of the fuel gas takes place particularly effectively at a low technical outlay. It must be remembered, in this case, that, for a particularly low overall height of the steam generator, the nitrogen removal device for fuel gas may be designed for an approximately vertical flow of the fuel gas from the top downward. It is thereby possible for the liquid necessary in the SCR method, together with ammonia fractions, to be injected in the main flow direction of the fuel gas, with the result that the nitrogen removal device has a particularly small vertical extent.
However, in a steam generator with a combustion chamber, through which fuel gas can flow in an approximately horizontal main flow direction, the fuel gases, after leaving the horizontal gas flue, flow downward in the vertical gas flue. In order to cause the fuel gas to flow approximately vertically from the top downward in the nitrogen removal device for fuel gas, it is therefore necessary to have a duct for the fuel gas, in which the fuel gas is guided from the bottom upward downstream of the vertical gas flue on the outlet side, in order then to enter the nitrogen removal device for fuel gas, through which said fuel gas is capable of flowing from the top downward. This additional duct is not desirable when the vertical gas flue is designed for an approximately vertical flow of the fuel gas from the bottom upward and the nitrogen removal device provided for the fuel gas is designed for an approximately vertical flow of the fuel gas from the top downward.
Advantageously, the purified flue gas leaving the nitrogen removal device for fuel gas can be used for the heating of air in an air preheater. The air preheater should in this case be arranged directly below the nitrogen removal device for fuel gas in a particularly space-saving way. The preheated air is to be supplied to the burners of the steam generator for the combustion of the fossil fuel. When hot air, in contrast to cold air, is supplied to the burners during the combustion of the fossil fuel, the overall efficiency of the steam generator rises.
The nitrogen removal device for fuel gas advantageously comprises a DeNOx catalyst. This is because a reduction in the nitrogen oxides of the fuel gas leaving the steam generator can then be carried out in a particularly simple way, for example by way of the method of selective catalytic reduction.
The containment walls of the combustion chamber are advantageously formed from vertically arranged evaporator tubes which are welded to one another in a gastight manner and a number of which are in each case capable of being acted upon in parallel by flow medium.
Advantageously, one containment wall of the combustion chamber is the end wall and two containment walls of the combustion chamber are the side walls, The side walls in each case are subdivided into a first group and a second group of evaporator tubes. The end wall and the first group of evaporator tubes are capable of being acted upon in parallel by a flow medium and, on the flow-medium side, the preceding second group of evaporator tubes are capable of being acted upon in parallel by the flow medium. Therefore, particularly favorable cooling of the end wall is thereby ensured.
Advantageously, the evaporator tubes capable in each case of being acted upon in parallel by the flow medium are, on the flow-medium side, preceded by a common inlet header system and followed by a common outlet header system. A steam generator designed in this configuration allows reliable pressure compensation between the parallel-connected evaporator tubes and therefore a particularly favorable distribution of the flow medium during the flow through the evaporator tubes.
In a further advantageous refinement, the inside tube diameter of a number of the evaporator tubes of the combustion chamber is selected as a function of the respective position of the evaporator tubes in the combustion chamber. The evaporator tubes in the combustion chamber may thereby be adapted to a heating profile predeterminable on the gas side. By the influence brought about thereby on the flow through the evaporator tubes, temperature differences at the outlet of the evaporator tubes of the combustion chamber are kept low in a particularly reliable way.
For a particularly good transmission of the heat of the combustion chamber to the flow medium guided in the evaporator tubes, a number of the evaporator tubes advantageously have on the inside thereof ribs forming a multiflight thread. In this case, advantageously, a pitch angle xcex1 between a plane perpendicular to the tube axis and the flanks of the ribs arranged on the tube inside is smaller than 60xc2x0, preferably smaller than 55xc2x0.
To be precise, in a heated evaporator tube designed as an evaporator tube without internal ribbing, what may be referred to as a smooth tube, the wetting of the tube wall, necessary for a particularly good heat transmission, may no longer be maintained beyond a specific steam content. In the absence of wetting, there may be a tube wall which is dry at particular points. The transition to a dry tube wall of this type leads to a kind of heat transmission crisis with an impaired heat transmission behavior, so that, in general, the tube-wall temperatures at this point rise particularly sharply.
In an internally ribbed tube, however, as compared with a smooth tube, this heat transmission crisis occurs only in the case of a steam mass content  greater than 0.9, that is to say just before the end of evaporation. This is attributable to the swirl which the flow experiences due to the spiral ribs. By virtue of the differing centrifugal force, the water fraction is separated from the steam fraction and is pressed onto the tube wall. The wetting of the tube wall is thereby maintained up to high steam contents, so that high flow velocities prevail even at the location of the heat transmission crisis. This gives rise, despite the heat transmission crisis, to a good heat transmission and, consequently, to low tube-wall temperatures.
A number of the evaporator tubes of the combustion chamber advantageously have the capability for reducing the throughflow of the flow medium. In this case, it proves to be particularly beneficial if the capability is designed as throttle devices. Throttle devices may, for example, be fittings which are built into the evaporator tubes and which reduce the tube inside diameter at a point within the respective evaporator tube.
In this case, it also proves advantageous to have the capability for reducing the throughflow in a line system which includes a plurality of parallel lines and through which flow medium can be supplied to the evaporator tubes of the combustion chamber. At the same time, the line system may also precede an inlet header system of parallel evaporator tubes capable of being acted upon by flow medium. In this case, for example, throttle fittings may be provided in one line or in a plurality of lines of the line system. By such means for reducing the throughflow of the flow medium through the evaporator tubes, the throughput of the flow medium through individual evaporator tubes may be adapted to the respective heating of these in the combustion chamber. As a result, in addition, temperature differences of the flow medium at the outlet of the evaporator tubes may be kept particularly low in a particularly reliable way.
The side walls of the horizontal gas flue and/or of the vertical gas flue are advantageously formed from vertically arranged steam generator tubes which are welded to one another in a gastight manner and a number of which are in each case capable of being acted upon in parallel by flow medium.
Adjacent evaporator or steam generator tubes are advantageously welded to one another in a gastight manner via metal bands, what may be referred to as fins. The fin width influences the introduction of heat into the steam generator tubes. The fin width is therefore adapted, preferably as a function of the position of the respective evaporator or steam generator tubes in the steam generator, to a heating and/or temperature profile predeterminable on the gas side. In this case, the predetermined heating and/or temperature profile may be a typical heating and/or temperature profile determined from empirical values or else a rough estimation, such as, for example, a stepped heating and/or temperature profile. By way of the suitably selected fin widths, it is possible, even in the case of widely varying heating of different evaporator or steam generator tubes, to achieve an introduction of heat into all the evaporator or steam generator tubes, in such a way that temperature differences at the outlet of the evaporator or steam generator tubes are kept particularly low. Premature material fatigues are reliably prevented in this way. The steam generator consequently has a particularly long useful life.
The horizontal gas flue advantageously has arranged in it a number of superheater heating surfaces, the tubes of which are arranged approximately transversely to the main flow direction of the fuel gas and are connected in parallel for a throughflow of the flow medium. These superheater heating surfaces, arranged in a suspended form of a construction and also designated as bulkhead heating surfaces, are heated predominantly convectively and follow the evaporator tubes of the combustion chamber on the flow-medium side. A particularly beneficial utilization of the fuel-gas heat is thereby ensured.
Advantageously, the vertical gas flue has a number of convection heating surfaces which are formed from tubes arranged approximately transversely to the main flow direction of the fuel gas. The tubes of a convection heating surface are in this case connected in parallel for a throughflow of the flow medium. These convection heating surfaces, too, are heated predominantly convectively.
In order, furthermore, to ensure a particularly full utilization of the heat of the fuel gas, the vertical gas flue advantageously has an economizer. Advantageously, the burners are arranged on the end wall of the combustion chamber, that is to say on that containment wall of the combustion chamber which is located opposite the outflow orifice to the horizontal gas flue.
A steam generator designed in this way can be adapted particularly simply to the burnup length of the fuel. The burnup length of the fossil fuel is understood as meaning, in this context, the fuel-gas velocity in the horizontal direction at a specific mean fuel-gas temperature, multiplied by the burnup time tA of the fossil fuel. The maximum burnup length for the respective steam generator is obtained in this case at the steam power output of the steam generator under full load, what may be referred to as full-load operation of the steam generator. The burnup time tA, in turn, is the time which, for example, a coal dust grain requires in order to burn up completely at a specific mean fuel-gas temperature.
In order to keep material damage and undesirable contamination of the horizontal gas flue, for example due to the introduction of molten ash at high temperature, particularly low, the length L of the combustion chamber, defined by the distance between the end wall and the inlet region of the horizontal gas flue, is advantageously at least equal to the burnup length of the fuel during full-load operation of the steam generator. This length L of the combustion chamber will generally be greater than the height of the combustion chamber, measured from the funnel top edge to the combustion chamber ceiling.
In an advantageous refinement, for the particularly favorable utilization of the combustion heat of the fossil fuel, the length L (given in m) of the combustion chamber is selected as a function of the BMCR value W (given in kg/s) of the steam generator, the burnup time tA (given in s) of the fuel and the outlet temperature TBRK (given in xc2x0C.) of the fuel gas from the combustion chamber. BMCR stands for boiler maximum continuous rating and gives the term conventionally used internationally for the maximum continuous power output of a steam generator. This also corresponds to the design power output, that is to say the power output during full-load operation of the steam generator. In this case, with a given BMCR value W of the steam generator, approximately the higher value of the two functions (I) and (II) applies to the length L of the combustion chamber:
L(W, tA)=(C1+C2xc2x7W)xc2x7tAxe2x80x83xe2x80x83(I)
and
L(W,TBRK)=(C3xc2x7TBRK+C4)W+C5(TBRK)2+C6xc2x7TBRKC7xe2x80x83xe2x80x83(II)
where
C1=8 m/s and
C2=0.0057 m/kg and
C3=xe2x88x921.905xc2x710xe2x88x924 (mxc2x7s)/(kgxc2x0C.) and
C4=0.286 (sxc2x7m)/kg and
C5=3xc2x710xe2x88x924 m/(xc2x0C.)2 and
C6=xe2x88x920.842 m/xc2x0C. and
C7=603.41 m.
xe2x80x9cApproximatelyxe2x80x9d is understood to mean in this case a permissible deviation of +20%/xe2x88x9210% from the value defined by the respective function.
The advantages achieved by way of the invention are. For example, that the steam generator has a particularly low space requirement on account of the horizontal combustion chamber and of the vertical gas flue designed for an approximately vertical flow direction of the fuel gas from the bottom upward. This particularly compact form of construction of the steam generator makes it possible, when the steam generator is incorporated into a steam turbine plant, to have particularly short connecting tubes from the steam generator to the steam turbine.